This invention relates to a fluid operated steering system having at least one fluid servomotor from which operating fluid is discharged under control of a relief valve to control the maximum pressure available for steering power purposes.
The present invention is applicable to power assisted steering mechanisms having a mechanical connection between the hand steering wheel and dirigible wheels of a vehicle and to hydrostatic steering mechanisms wherein the steering forces are transmitted by pressurized liquid. In hydro-steering mechanisms of one well known type, the two operating chambers of a fluid servomotor are connected to one relief valve for discharge of operating liquid. The relief valve functions as a shock absorber to prevent overload because of excessively high pressure that may otherwise occur as a result of road influences. In hydraulic power assisted steering mechanisms, the operating chambers of the servomotor are connected to a valve of a different type for controlling discharge of the operating liquid. The latter type of steering mechanism is illustrated, for example, on page 10 of a publication "ZF Steering Gears with Accessories and Available Pumps", published by the Assignee of the present application in West Germany identified as G 7,000, P. The discharge valves shown therein serve to connect the appropriate pressure chamber of the servomotor to a discharge outlet as the steering gear approaches its limit of movement to thereby avoid overtravel of the piston or of any portion of the steering linkage connected thereto under the impetus of full hydraulic power. The overtravel problem dealt with by such discharge valves differs, however, from the problem with which the present invention is concerned.
The present invention relates to the problem of steering relatively heavy commercial vehicles or the like while in a substantially stationary condition encountering high steering resistance as a result of road contact. In such cases, the steering of relatively stationary wheels requires hydraulic power of high magnitude. When provided with a powerful hydraulic steering system to meet such high power requirements, the vehicle driver is often tempted to oversteer since low forces on the hand steering wheel are sensed during rapid straight ahead travel. The vehicles are not, however, sufficiently dimensioned or structured for the high resulting stresses. In order to prevent damage to the vehicle, auxiliary power steering systems having a relatively low hydraulic power output have heretofore been utilized deliberately providing insufficient steering capacity for the vehicle in the standing position.